1. Field of the Invention
The invention relates to gas separation adsorption operations. More particularly, it relates to a process for producing mixed-cation zeolitic adsorbents for use in such adsorption operations.
2. Description of the Prior Art
Synthetic zeolite molecular sieves are used commercially as adsorbents in pressure swing adsorption (PSA) processes for the separation and purification of glass. Such materials, e.g. 13X and 5X materials, are complex aluminosilicates that have open framework structures that present a large, effective surface for the adsorption of small gas molecules. The positive charge deficiency due to the presence of aluminum in tetrahedral sites is compensated by the presence of alkaline or alkaline earth ions. The compensating ion is typically Na.sup.+, but the open nature of the structure makes the ions accessible to ion exchange with other cations in aqueous solution.
For the production of nitrogen by PSA-air separation processes, it has been found that zeolites containing Li.sup.+ ions, or mixtures of Li.sup.+ ions with alkaline earth cations such as Ca.sup.++ or Mg.sup.++, have particularly desirable properties. The Chao et al. patents, U.S. Pat. Nos. 5,174,979 and 5,413,625, disclose such mixed zeolites and the use thereof in gas separation operations. Li.sup.+ zeolites are prepared from the corresponding Na.sup.+ zeolites by ion exchange. Thus, a concentrated aqueous solution of Li.sup.+ Cl.sup.- is passed through a column containing the Na.sup.+ zeolite. The Na.sup.+ ions are displaced by the Li.sup.+ ions to produce the desired Li.sup.+ zeolite. Since zeolites generally have a greater affinity for the Na.sup.+ ion than for the Li.sup.+ ion, a considerable quantity of strong Li.sup.+ Cl.sup.- solution is required, and the spent liquor from this exchange contains a high concentration of both Na.sup.+ and Li.sup.+ ions. The lithium contained in the spend liquor is too valuable to waste and must be recovered from the spent liquor in practical commercial operations.
The production of multi-cation, or mixed cation, zeolites, such as LiCaX or LiMgX, particularly when trace amounts of Ca.sup.++ or Mg.sup.++ are desired, presents operating difficulties. The zeolite has a much greater affinity for the alkaline earth ions than for Li.sup.+. To incorporate traces of Ca.sup.++ into a bed of Li.sup.+ zeolite by equilibration with a solution requires the passage of a very large quantity of very dilute Ca.sup.++ solution through the bed. This approach is generally impractical in commercial operations. If a smaller quantity of solution, with a higher concentration of Ca.sup.++, is passed through a bed of Li.sup.+ zeolite, the Ca.sup.++ will displace the Li.sup.+ from the entrance end of the bed, and the resulting ion-exchanged bed will have a non-uniform distribution of cations therein. As such a non-uniform bed distribution is not desirable for PSA operations, an alternative process is desired in the art for producing multi-cation zeolites.
Synthetic zeolites are usually made with a single species of cation, typically Na.sup.+ as indicated above. In the changing of the cations in a zeolite by the displacing of the original cations on the zeolite with other desired cations, the salt solution containing the desired cations is passed through a bed of zeolite at a particular concentration and flow rate depending on the different affinities of the different cations. The progress of the ion exchange operation depends on the concentration and flow rate of the ion exchange solution through the zeolitic bed. To produce a mixed-cation zeolite, the solution can be prepared with cation salt concentrations that are in equilibrium with the desired concentrations of cations in the zeolite. Unfortunately, this process is not practical when the exchange affinities of the cations are very different, as they are for Li.sup.+ and Ca.sup.++. As indicated above, this process would require a very large quantity of solution that is highly concentrated in Li.sup.+ and very dilute in Ca.sup.++.
The preparation of zeolites, and the properties of zeolites exchanged with both Li.sup.+ and Ca.sup.++ or Si.sup.++, has been described in the Chao et al. patents referred to above. The Li.sup.+ zeolite is first prepared, on a laboratory scale, and divalent cations are then added in a solution containing a concentration of divalent cations that is much higher than the desired final equilibrium concentration, but with the total quantity of divalent cations being close to that desired in the final product. For the reasons indicated above, improved processing techniques are desired for large-scale, commercial production operations.
While it is relatively easy to displace all or nearly all of the Li.sup.+ by divalent ions, i.e. M.sup.++ ions, which are strongly taken up by the zeolite, it is much more difficult to obtain a uniform product, particularly one containing a minor fraction of M.sup.++ in addition to Li.sup.+. The divalent cations are quickly taken up by the zeolite that is first contacted, i.e. at the feed end of the bed, thus leading to a non-uniform product as indicated above. The problem of mixed cation zeolite bed non-uniformity is one for which further development in the adsorption field is desired.
It is an object of the invention, therefore, to provide an improved process for the production of mixed-cation zeolites.
It is another object of the invention to provide a process for the production of mixed-cation zeolites with reduced loss of valuable cations.
It is a further object of the invention to provide a process for the production of a uniform bed of mixed-cation zeolites.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.